In trucks and similar vehicles, a source of a pressurized fluid such as compressed air is generally provided and actuators are used which include both a service portion and an auxiliary portion. The service portion is used for the normal application of the brakes in response to a service pressure of variable magnitude developed from the source pressure through an operator-controlled valve. The auxiliary portion is operative in emergency or other conditions in which the source of fluid pressure fails or is otherwise removed. Generally, it includes a powerful spring which is held in a compressed energy storing condition by source pressure, being released to apply the brakes.
Various constructions for such actuators are disclosed in U.S. Pat. Nos. 2,809,723; 3,117,496; 3,131,609; 3,182,566; 3,302,530; 3,331,291; 3,502,003; 3,581,627; 3,636,822; and 3,908,520.
In such constructions, the service portion includes a diaphragm having an annular flexible portion between the inside of a housing and the periphery of an actuating member which is connected to the rearward end of a rod. The forward end of the rod is connected to a control element of a brake assembly and when service pressure is applied against the rearward side of the actuating member and the diaphragm, the rod drives the control element forwardly to engage brake elements with a drum or disc and to apply a braking force to the vehicle. In one type of actuator the auxiliary portion includes a diaphragm and actuating member similar to those in the service portion. In another type, the auxiliary portion includes a piston having peripheral bearing and seal means in slidable engagement with a cylindrical internal housing surface. Both types have been used with generally satisfactory results.